In recent years, with the trend toward higher integration and higher performance of semiconductor integrated circuits (hereinafter “LSI circuits”), new fine-processing techniques are on development. Chemical mechanical polishing (hereinafter “CMP”) is one of them, which is a technique frequently used in LSI circuit fabrication steps, in particular, in the planarizing of interlayer insulating films, the formation of metal plugs and the formation of buried wiring, in the step of forming multi-layer wiring. This technique is disclosed in, e.g., U.S. Pat. No. 4,944,836.
Recently, aiming at higher performance of LSI circuits, it is also attempted to use copper alloys as a wiring material. The copper alloys, however, make it difficult to perform the fine processing by dry etching that has frequently been used in the formation of conventional aluminum alloy wiring. Accordingly, what is called damascene process is employed in which copper or its alloy thin film is deposited on an insulating film having grooves which are formed previously, the film standing buried in the grooves, and the copper alloy thin film other than that on the grooves are removed by CMP to form buried wiring. This technique is disclosed, e.g., in Japanese Patent Application Laid-open No. 2-278822.
In a common method for CMP of metals, a polishing pad is stuck onto a circular polishing surface plate (platen), the surface of the polishing pad is soaked with a polishing slurry for metal, the surface of a substrate on which a metal film has been formed is pressed against the pad surface, the polishing platen is rotated in the state a preset pressure (hereinafter “polishing pressure”) is applied from its back side, and hills of the metal film are removed by mechanical friction between the polishing slurry and the hills of the metal film.
Polishing slurries for metal which are used in CMP are commonly comprised of an oxidizing agent and solid abrasive particle or powder and also an oxidized-metal dissolving agent and a protective-film forming agent which are optionally further added. What is considered to be basic mechanism is that the metal film surface is first oxidized by oxidation and the oxide layer thus formed is scraped off by the solid abrasive grains. The oxide layer at valleys of the metal surface does not so much come in touch with the polishing pad, and the effect of scrape-off by solid abrasive grains does not extend thereto, so that with progress of CMP, hills of the metal layer are removed and the metal member surface become smooth. Details of the matter are disclosed in Journal of Electrochemical Society, Vol. 138, No. 11 (published 1991), pages 3460-3464.
It is considered that the effect of scrape-off by solid abrasive grains is enhanced as long as the grains of a metal oxide scraped off by the solid abrasive grains have been dissolved in the polishing slurry by the aid of the oxidized-metal dissolving agent. If, however, the oxide layer at valleys of the metal film surface is also dissolved (hereinafter “etched”) until the metal film surface becomes uncovered, the metal film surface is further oxidized by the oxidizing agent. If this is repeated, the oxide layer at valleys may unwantedly be etched further, resulting in a loss of the effect of smoothing. There is such a possibility. In order to prevent it, a protective-film forming agent is further added. It is important to well balance the effects attributable to the oxidized-metal dissolving agent and protective-film forming agent, and it is desired that the oxide layer of the metal film surface is not so much etched, that the grains of the oxide layer scraped off are dissolved in a good efficiency and that the polishing by CMP is at a high rate.
Thus, adding such oxidized-metal dissolving agent and protective-film forming agent so as to add an effect of chemical reaction brings about an improvement in CMP rate (i.e., polishing rate attributable to CMP), and also can provide the effect of less damaging the metal film surface subjected to the CMP.
However, when the buried wiring is formed by CMP using the conventional polishing slurry for metal, containing solid abrasive grains, there are problems such that (1) a phenomenon may take place in which the middle portion of the surface of buried metal wiring is isotropically corroded to become hollow like a dish (hereinafter “dishing”), (2) polishing mars (scratches) due to the solid abrasive grains may occur, (3) a complicated cleaning process is required for removing any solid abrasive grains remaining on the substrate surface after polishing, and (4) the initial cost of the solid abrasive grains themselves and the disposal of waste liquid brings about a high cost.
In order to keep the dishing from occurring and the copper alloy from being corroded during polishing and to form highly reliable LSI wiring, a method making use of a polishing solution for metal which contains an oxidized-metal dissolving agent comprised of aminoacetic acid (glycine) or amidosulfuric acid and benzotriazole (hereinafter “BTA”) is proposed. This technique is disclosed in, e.g., Japanese Patent Application Laid-open No. 8-83780.
The BTA, however, has so high a protective-film forming effect that it may cause a great decrease in not only etching rate but also polishing rate. Hence, it is demanded to use in the polishing solution for metal a protective-film forming agent that does not cause any decrease in CMP rate.